The present invention concerns a novel process for the preparation of precipitated silica, in particular precipitated in the form of a powder, substantially spherical spherules, or granules, and their application as a reinforcing filler for elastomers.
Precipitated silica has long been used as a white reinforcing filler for elastomers, in particular for tyres.
However, like all reinforcing fillers, it must capable of ready manipulation in, and above all incorporation into the mixtures.
It is generally known that, in order to produce optimal reinforcing properties from a filler, the latter must be present in the elastomer matrix a final form which is both as finely divided as possible and distributed as homogeneously as possible. These conditions can only be satisfied if the filler can be extremely easily incorporated into the matrix during mixing with the elastomer (filler incorporability) and can disaggregate or deagglomerate to a very fine powder (filler disaggregation), and if the powder produced by the disaggregation process described can itself disperse perfectly and homogeneously in the elastomer (powder dispersion).
Further, the silica particles have an annoying tendency to agglomerate among themselves in the elastomer matrix because of mutual attraction. These silica/silica interactions limit the reinforcing properties to a level which is far lower than that which could theoretically be achieved if all the silica/elastomer interactions which could be formed during the mixing operation were formed (this theoretical number of silica/elastomer interactions is known to be directly proportional to the external surface area of the silica used).
These silica/silica interactions also tend to increase the rigidity and consistency of the mixtures in the uncured state, making their use more difficult.
It is difficult to find fillers which, while having a relatively large size, readily disperse in elastomers.
The present invention aims to overcome the drawbacks described above and resolve the difficulty mentioned above.
More precisely, it provides a novel process for the preparation of precipitated silica which advantageously has improved dispersion (and deagglomeration) abilities and/or reinforcing properties, in particular when used as a filler for elastomers, to provide the latter with an excellent compromise between their different mechanical properties.
The invention also concerns precipitated silicas, preferably in the form of a powder, substantially spherical spherules or, optionally, granules and which, while relatively large in size, have highly satisfactory dispersion (and deagglomeration) abilities. They also have improved reinforcing properties.
Finally, the invention concerns the use of said precipitated silicas as reinforcing fillers for elastomers.
One of the objects of the invention is thus to provide a process for the preparation of a precipitated silica comprising reacting an alkali metal M silicate with an acidifying agent to produce a suspension of a precipitated silica, then separating and drying said suspension, characterised in that precipitation is carried out as follows:
(i) forming an initial seed comprising a portion of the total amount of the alkali metal M silicate used in the reaction, the concentration of silicate expressed as SiO2 in said seed being less than 20 g/l,
(ii) adding acidifying agent to said initial seed until at least 5% of the quantity of M2O present in said initial seed is neutralised,
(iii) simultaneously adding acidifying agent and the remaining quantity of alkali metal M silicate to the reaction medium such that the ratio of the quantity of silicate added (expressed as SiO2)/quantity of silicate present in the initial seed (expressed as SiO2), the consolidation ratio, is greater than 4 and at most 100.
It has thus been discovered that a very low concentration of silicate expressed in SiO2 in the initial seed and an appropriate consolidation ratio during the simultaneous addition step constitute important conditions for ensuring that the products obtained have excellent properties.
It should be noted that, in general, the process concerned is a process for the synthesis of precipitated silica, ie., an acidifying agent is reacted with an alkali metal M silicate.
The choice of acidifying agent and silicate is made in known fashion. The acidifying agent is usually a strong mineral acid such as sulphuric acid, nitric acid or hydrochloric acid, or an organic acid such as acetic acid, formic acid or a carboxylic acid.
The silicate can be in any known form such as a metasilicate, disilicate or, advantageously, an alkali metal M silicate where M is sodium or potassium.
In general, the acidifying agent used is sulphuric acid, and the silicate used is sodium silicate.
When using sodium silicate, this generally has a SiO2/Na2O molar ratio of between 2 and 4, more particularly between 3.0 and 3.7.
Specifically, precipitation is carried out using the following steps of the process of the invention.
Firstly, a seed containing silicate is formed. The quantity of silica present in the initial seed advantageously only represents a portion of the total quantity of silicate used in the reaction.
According to an essential feature of the preparation process of the invention, the concentration of silicate in the initial seed is less than 20 g of SiO2 per liter.
This concentration can be at most 11 g/l, optionally at most 8 g/l.
Particularly when the separation carried out at the end of the process of the invention comprises filtration using a filter press, this concentration is preferably at least 8 g/l, in particular between 10 and 15 g/l, for example between 11 and 15 g/l; the subsequent drying step in the process of the invention is then advantageously carried out by atomisation using a spray diffuser.
The conditions imposed on the silicate concentration in the initial seed partially determine the characteristics of the silicas obtained.
The initial seed may contain an electrolyte. Nevertheless, it is preferable that no electrolyte is used during the preparation process of the invention; in particular, the initial seed preferably contains no electrolyte.
The term xe2x80x9celectrolytexe2x80x9d has its normal meaning here, ie., it signifies any ionic or molecular substance which, when in solution, decomposes or dissociates to form ions or charged particles. A salt from the group formed by alkali metal and alkaline-earth metal salts can be cited as an electrolyte, in particular the salt of the starting metal silicate and the acidifying agent, for example sodium sulphate in the case of the reaction of sodium silicate with sulphuric acid.
The second step consists in adding the acidifying agent to the seed with the composition described above.
Thus, in the second step, the acidifying agent is added to said initial seed until at least 5%, preferably at least 50%, of the quantity of M2O present in said initial seed is neutralised.
Preferably, the acidifying agent is added to said initial seed in said second step until 50% to 99% of the quantity of M2O present in said initial seed is neutralised.
The acidifying agent can be dilute or concentrated; the concentration can be between 0.4 and 36N, for example between 0.6 and 1.5N
In particular, when the acidifying agent is sulphuric acid, its concentration is preferably between 40 and 180 g/l, for example between 60 and 130 g/l.
When the desired concentration of neutralised M2O has been reached, simultaneous addition (step (iii)) of the acidifying agent and a quantity of alkali metal M silicate is commenced so that the consolidation ratio, ie., the ratio of the quantity of silicate added (expressed as SiO2)/quantity of silicate present in the initial seed (expressed as SiO2) is greater than 4 and at most 100.
In one embodiment of the invention, simultaneous addition of the acidifying agent and quantity of alkali metal M silicate is carried out such that the consolidation ratio is more particularly between 12 and 100, preferably between 12 and 50, most particularly between 13 and 40.
In a further embodiment of the invention, simultaneous addition of the acidifying agent and quantity of alkali metal M silicate is carried out such that the consolidation ratio is greater than 4 and less than 12, preferably between 5 and 11.5, more particularly between 7.5 and 11. This embodiment is generally employed when the silicate concentration in the initial seed is at least 8 g/l, in particular between 10 and 15 g/l, for example between 11 and 15 g/l.
Preferably, the quantity of acidifying agent added during the totality of step (iii) is such that 80% to 99%, for example 85% to 97% of the quantity of M2O added is neutralised.
In step (iii), it is possible to carry out the simultaneous addition step of the acidifying agent and the silicate at a first reaction medium pH of pH1, then at a second reaction medium pH of pH2, such that 7 less than pH2 less than pH1 less than 9.
The acidifying agent used during step (iii) can be diluted or concentrated: the concentration can be between 0.4 and 36 N, for example between 0.6 and 1.5 N.
In particular, when the acidifying agent is sulphuric acid, its concentration is preferably between 40 and 180 g/l, for example between 80 and 130 g/l.
In general, the alkali metal M silicate added during step (iii) has a concentration, expressed as silica, of between 40 and 330 g/l, for example between 60 and 300 g/l, in particular between 60 and 250 g/l.
The precipitation reaction itself is terminated when the remaining quantity of silicate has been added.
It may be advantageous, particularly following the simultaneous addition step, to mature the reaction medium over a period of 1 to 60 minutes, in particular 5 to 30 minutes.
Finally, it is desirable to add a supplemental quantity of acidifying agent to the reaction medium in a subsequent step following precipitation and prior to maturing. This addition step is generally carried out until the reaction medium reaches a pH of between 3 and 6.5 is reached, preferably between 4 and 5.5. It allows all the M2O added during step (iii) to be neutralised and regulates the final pH of the silica to the desired value for the given application.
The acidifying agent used in this addition step is generally identical to that used during step (iii) of the process of the invention.
The temperature of the reaction medium is normally between 60xc2x0 C. and 98xc2x0 C.
Preferably, during step (ii) the acidifying agent is added at a temperature of between 60xc2x0 C. and 96xc2x0 C. to the initial seed.
In a further embodiment of the invention, the reaction is carried out at a constant temperature between 70xc2x0 C. and 90xc2x0 C. (particularly when the consolidation ratio is greater than 4 and less than 12) or between 75xc2x0 C. and 96xc2x0 C. (particularly when the consolidation ratio is between 12 and 100).
In a still further embodiment of the invention, the temperature at the end of the reaction is higher than the temperature at the beginning of the reaction: thus, the starting temperature is preferably maintained between 70xc2x0 C. and 90xc2x0 C. (particularly when the consolidation ratio is greater than 4 and less than 12) or between 70xc2x0 C. and 96xc2x0 C. (particularly when the consolidation ratio is between 12 and 100), then the temperature is increased over several minutes during the course of the reaction, preferably to between 75xc2x0 C. and 98xc2x0 C., for example between 80xc2x00 C. and 90xc2x0 C. (particularly when the consolidation ratio is greater than 4 or less than 12) or between 80xc2x0 C. and 98xc2x0 C. (particularly when the consolidation ratio is between 12 and 100), and kept at this value until the reaction is finished.
A silica slurry is produced after the operations just described. This is then separated (liquid-solid separation). Separation generally consists of filtration followed by washing if required. If filtration can be carried out using any convenient method (for example using a filter press, band filter or rotary vacuum filter), it is advantageously carried out using a filter press when the silicate concentration in the initial seed is at least 8 g/l (and less than 20 g/l), in particular between 10 and 15 g/l, for example between 11 and 15 g/l.
The recovered suspension of precipitated silica (filtration cake) is then dried.
Drying can be carried out using any known means.
Drying is preferably effected by atomisation.
Any suitable atomiser can be used, in particular centrifugal driers, spray diffusers, pressurised liquid sprays or double fluid sprays.
Drying is advantageously effected by atomisation using a spray diffuser when the silicate concentration in the initial seed is at least 8 g/l (and less than 20 g/l), in particular between 10 and 15 g/l, for example between 11 and 15 g/l.
The precipitated silica which can be obtained under these conditions of silicate concentration using a filter press and a spray diffuser is normally in the form of substantially spherical spherules, preferably with an average size of at least 80 xcexcm.
In a still further embodiment of the process of the invention, the suspension to be dried has a dry matter content of more than 15% by weight, preferably greater than 17% by weight, for example greater than 20% by weight. Drying is preferably carried out using a spray diffuser.
The precipitated silica which can be obtained using this embodiment is normally in the form of substantially spherical spherules, preferably with an average size of at least 80 xcexcm.
This dry matter content can be produced by direct filtration using a suitable filter in particular a filter press) to give a filter cake with the correct content. An alternative method consists in adding dry material, for example powdered silica, to the cake in a final step of the process, following filtration, to produce the required content.
It should be noted that it is well known that the cake obtained is generally not in an atomisable condition principally because the viscosity is too high.
The cake is then disintegrated using known techniques. This operation can be carried out by passing the cake through a colloidal or ball mill. In addition, the viscosity of the suspension to be atomised can be reduced by adding aluminium, in particular in the form of sodium aluminate, during the process, as described in French patent application FR-A-2 536 380, whose subject matter is hereby incorporated. The addition can in particular be made at the disintegration stage.
A milling step can follow the drying step, particularly when the recovered product has been obtained by drying a suspension with a dry matter content of more than 15% by weight. The precipitated silica which can then be obtained is generally in the form of a powder, preferably with an average size of at least 15 xcexcm, in particular between 15 and 60 xcexcm, for example between 20 and 45 xcexcm.
Once they have been milled to the desired granulometry, the products can be separated from any products which do not conform to the average, for example using vibrating sieves of suitable mesh size, and the non conforming products which are recovered can be returned to the milling step.
In a further embodiment of the process of the invention, the suspension to be dried has a dry matter content of less than 15% by weight. Drying is generally carried out in a centrifugal drier. The precipitated silica which can then be obtained is generally in the form of a powder, preferably with an average size of at least 15 xcexcm, in particular between 30 and 150 xcexcm, for example between 45 and 120 xcexcm.
Disintegration can also be carried out.
Finally, the dried product (particularly from a suspension with a dry matter content of less than 15% by weight) or milled product can be submitted to an agglomeration step in a still further embodiment of the process of the invention.
The term xe2x80x9cagglomerationxe2x80x9d means any process which binds together finely divided objects to form larger, mechanically resistant objects.
Particular processes are direct compression, wet granulation (ie., using a binder such as water, silica slurry, . . . ), extrusion and, preferably, dry compaction.
When using the latter technique, it can be of advantage to deaerate (also known as predensification or degassing) the powdered products before compaction to eliminate the air included in the products and ensure more regular compaction.
The precipitated silica which can be obtained from this embodiment of the invention is advantageously in the form of granules, preferably with dimensions of at least 1 mm, in particular between 1 and 10 mm.
Following the agglomeration step, the products can be calibrated to the desired size, for example by sieving, then packaged for future use.
One advantage of the powders and spherules of precipitated silica obtained using the process of the invention is that they can simply, efficiently and economically be formed into granules as described, particularly using conventional forming operations, such as granulation or compaction, and these operations do not cause deterioration which can mask or even destroy the excellent intrinsic reinforcing properties of these powders, as could be the case when using the conventional powders of the prior art.
The invention also relates to novel precipitated silicas having a good dispersion (and deagglomeration) ability and improved reinforcing properties, said silicas preferably being relatively large in size and generally being obtained using one of the embodiments of the process of the invention described above.
In the following description, the BET specific surface area was determined using the BRUNAUER-EMMET-TELLER method described in xe2x80x9cThe Journal of the American Chemical Societyxe2x80x9d, Vol.60. page 809, February 1938, corresponding to standard NFT 45007 (November 1987).
The CTAB specific surface area is the external surface area determined in accordance with standard NFT 45007 (November 1987) (5.12).
The DOP oil absorption value was determined in accordance with standard NFT 30-022 (March 1953) using dioctylphthalate.
The loose packing density (LPD) was measured in accordance with standard NFT-030100.
Finally, the pore volumes given were measured by mercury porosimetry. The pore diameters were calculated using the WASHBURN relation with an angle of contact theta of 130xc2x0 and a surface tension gamma of 484 dynes/cm (MICROMERITICS 9300 porosimeter).
The dispersibility and deagglomeration ability of the silicas of the invention was quantified using a specific deagglomeration test.
The deagglomeration test was carried out as follows:
agglomerate cohesion was measured by granulometric measurement (using a diffraction laser), carried out on a silica suspension which had been deagglomerated using ultrasound; the deagglomeration ability of the silica (breakdown of objects from 0.1 to several dozen microns) was thus measured. Ultrasound deagglomeration was effected using a VIBRACELL BIOBLOCK (600 W) sonificator equipped with a 19 mm diameter probe. Granulometric measurements were carried out using a diffraction laser on a SYMPATEC granulometer.
2 grams of silica were measured into a small beaker (height: 6 cm and diameter: 4 cm) and brought up to 50 grams by addition of deionised water: an aqueous suspension containing 4% of silica was thus formed which was homogenized for 2 minutes using a magnetic stirrer. Ultrasound deagglomeration was then carried out as follows: the probe was immersed to a depth of 4 cm and the output power was regulated to produce a 20% needle deviation on the power dial (corresponding to an energy dissipation of 120 watt/cm2 at the probe tip). Deagglomeration was carried out for 420 seconds. Granulometric measurement was then carried out after introducing a known volume (in ml) of the homogenised suspension into the granulometer cell.
The medial diameter xcfx8650 obtained was lower the greater the deagglomeration ability of the silica. The ratio (10xc3x97volume of suspension introduced in ml)/optical density of the suspension measured by granulometry, where the optical density is of the order of 20, was also measured. This ratio indicated the fines content, ie., the ratio of particles of less than 0.1 xcexcm which are not detected by the granulometer. This ratio, termed the ultrasound deagglomeration factor (FD) is higher when the silica has a higher deagglomeration ability.
A first embodiment of the invention provides a novel precipitated silica characterised in that it has the following properties:
a CTAB specific surface area (SCTAB) of between 140 and 240 m2/g, preferably between 140 and 225 m2/g, for example between 150 and 225 m2/g, in particular between 150 and 200 m2/g,
an ultrasound deagglomeration factor (FD) of more than 11 ml, for example more than 12.5 ml,
a medial diameter (xcfx8650), following ultrasound deagglomeration, of less than 2.5 xcexcm, in particular less than 2.4 xcexcm, for example less than 2.0 xcexcm.
A second embodiment of the invention provides a novel precipitated silica characterised in that it has the following properties:
a CTAB specific surface area (SCTAB) of between 140 and 240 m2/g, preferably between 140 and 225 m2/g, for example between 150 and 225 m2/g,
a pore distribution such that the pore volume constituted by pores with diameters of between 175 and 275 xc3x85 represents less than 50% of the pore volume constituted by pores with diameters of less than or equal to 400 xc3x85,
an ultrasound deagglomeration factor (FD) of more than 5.5 ml,
a medial diameter (xcfx8650), following ultrasound deagglomeration, of less than 5 xcexcm.
One feature of a silica in accordance with the second embodiment of the invention is its pore volume distribution, in particular the pore volume distribution constituted by pores with diameters of less than or equal to 400 xc3x85. This latter volume corresponds to the useful pore volume of fillers which are used to reinforce elastomers. Porogram analysis shows that silicas in accordance with the second embodiment of the invention have less than 50%, preferably less than 40% of their useful pore volume constituted by pores with diameters in the range 175 to 275 xc3x85.
Preferably, silicas in accordance with the second embodiment of the invention have the following properties:
an ultrasound deagglomeration factor (FD) of more than 11 ml, for example more than 12.5 ml, and/or
a medial diameter (xcfx8650), following ultrasound deagglomeration, of less than 4 xcexcm, for example less than 2.5 xcexcm.
The silicas of the invention generally have a BET specific surface area (SBET) of between 140 and 300 m2/g, in particular between 140 and 280 m2/g, for example between 150 and 270 m2/g.
In a further embodiment of the invention, the silicas have a SBET/SCTAB ratio of between 1.0 and 1.2, ie., the silicas have a low microporosity.
In a still further embodiment of the invention, the silicas have a SBET/SCTAB ratio of more than 1.2, for example between 1.21 and 1.4, ie., the silicas have a relatively high microporosity.
The silicas of the invention generally have a DOP oil absorption value of between 150 and 400 ml/100 g, more particularly between 180 and 350 ml/100 g, for example between 200 and 310 ml/100 g.
The silicas of the invention can be in the form of a powder, substantially spherical spherules or optionally granules, and are particularly characterised in that, while of relatively large size, they have remarkable deagglomeration ability and dispersibility and highly satisfactory reinforcing properties. They thus advantageously have a superior deagglomeration ability and dispersibility and a specific surface area and size which is identical to or close to those of prior art silicas.
Silica powders in accordance with the invention preferably have an average size of at least 15 xcexcm, for example, between 20 and 120 xcexcm or between 15 and 60 xcexcm (in particular between 20 and 45 xcexcm) or between 30 and 150 xcexcm (in particular between 45 and 120 xcexcm).
The loose packing density (LPD) of said powders is generally at least 0.17, for example between 0.2 and 0.3.
Said powders generally have a total pore volume of at least 2.5 cm3/g, more particularly between 3 and 5 cm3/g.
This produces a very good compromise between use and final mechanical properties in the vulcanised state.
Finally, they constitute particularly suitable precursors for the synthesis of granules as will be described below.
The substantially spherical spherules of the invention preferably have an average size of at least 80 xcexcm.
In certain embodiments of the invention, this average spherule size is at least 100 xcexcm, for example at least 150 xcexcm; it is generally at most 300 xcexcm and is preferably between 100 and 270 xcexcm. The average size is determined in accordance with standard NFxc3x9711507 (December 1970) by dry sieving and determination of the diameter corresponding to an accumulated residue of 50%.
The loose packing density (LPD) of said spherules is generally at least 0.17, for example between 0.2 and 0.34.
They generally have a total pore volume of at least 2.5 cm3/g, in particular between 3 and cm3/g.
As indicated above, a silica in the form of substantially spherical spherules, which are advantageously solid, homogeneous, of low pulverulence and with good flow characteristics, has a very good deagglomeration ability and dispersibility. It also has excellent reinforcing properties.
This type of silica also constitutes a highly suitable precursor for the synthesis of powders and granules in accordance with the invention.
The dimensions of the granules of the invention are preferably at least 1 mm, in particular between 1 and 10 mm, measured along the axis of their largest dimension (length).
Said granules can be in a variety of forms. Examples are spheres, cylinders, parallelepipeds, pellets, platelets or circular section or multilobed extrudates.
The loose packing density (LPD) of said granules is generally at least 0.27, up to 0.37.
They generally have a total pore volume of at least 1 cm3/g, more particularly between 1.5 and 2 cm3/g.
The silicas of the invention, in particular in the form of a powder, substantially spherical spherules or granules, are preferably prepared using a suitable embodiment of the process of the invention described above.
Silicas in accordance with the invention or prepared using the process of the invention have particular application in reinforcing natural or synthetic elastomers, in particular tyres. They provide the elastomers with an excellent compromise between their different mechanical properties, in particular a significant improvement in rupture or tear resistance and, in general, good abrasion resistance. In addition, these elastomers preferably heat up to a lesser extent.
The following examples illustrate she invention without in any way limiting its scope.